1. Field of the Invention
The present invention relates to a blowby gas returning device for returning blowby gas leaking from a combustion chamber of an engine to the combustion chamber by allowing the blowby gas to flow in a returning passage and an intake passage via a PCV valve.
2. Description of Related Art
Heretofore, some techniques of this type have been known as disclosed in for example JP2005-240605A, JP53-148639(1978)A, JP60-171915(1985)U, and JP2-3038(1990)U. Particularly, a device disclosed in JP2005-240605A is provided with a blowby gas passage between a crank case and an intake passage in an engine, and a PCV valve is placed in this blowby gas passage. This PCV valve is arranged to control a flow rate of blowby gas by a differential pressure between an inlet port side and an outlet port side of the valve. Here, the blowby gas contains unburned fuel constituent. When this blowby gas is returned to the combustion chamber of the engine, therefore, the fuel constituent contained in the blowby gas is burned together with the fuel normally supplied to the engine.
In the device of JP2005-240605A, however, the fuel constituent contained in the blowby gas may increase in concentration suddenly. In this case, when the blowby gas is returned to the combustion chamber, an air-fuel ratio in the engine may fluctuate to an over-rich side, causing disorder of air-fuel ratio control of the engine. During idling of this engine, especially, the blowby gas would significantly affect the air-fuel ratio. This may cause a change in engine rotational speed and deterioration in exhaust emission. Further, in a direct-injection engine arranged to directly inject fuel into engine cylinders, unburned fuel droplets sticking to a bore wall surface due to fuel injection may flow down along the wall surface and is mixed into a lubricant in an oil pan or receiver. This unburned fuel begins to vaporize when the temperature of the lubricant (oil temperature) rises to “40° C.” or more. When such gas is returned to the combustion chamber, accordingly, the air-fuel ratio may fluctuate, causing disorder of the air-fuel ratio control. Here, as to the engine air-fuel ratio control, learning control configured to learn an air-fuel ratio correction value according to an operating state of an engine and reflect it in the control. This type of learning control can deal with the influence of blowby gas returned to the combustion chamber to some extent. However, when the concentration and the volume of the fuel constituent of the blowby gas exceed permissible ranges in the learning control, the learning control could not appropriately respond thereto, thus causing the air-fuel ratio to fluctuate to an over-rich side, leading to disorder of the air-fuel ratio control.